1. Field of the Invention
The present invention broadly relates to an equipment such as reproducing apparatus, facsimile and the like having a document reading apparatus incorporating a photoelectric conversion element and, more particularly, to a shading correction device for use in the document reading apparatus in equipments of the type mentioned above.
2. Description of the Prior Art:
Such a recording device has been known and used broadly as having a lamp for illuminating a document, an optical system including a reflecting mirror and lenses, photoelectric conversion element such as a solid state image sensor or photodiode array to which the light reflected by the document is applied through the optical system to form an image, a stylus electrode or the like by which an electrostatic latent image is formed in accordance with the electric signal derived from the image sensor and means for developing the electrostatic latent image to record the image.
In this type of recording apparatus, the output from the image sensor is often made non-uniform even when the density of the document surface is uniform. Particularly, the output at the marginal edge portions becomes lower than that at the central region. The reproduced image is darkened in the area of lower output to make the density of the reproduced image non-uniform. This phenomenon, generally referred to as "shading" is attributable to the following reasons.
(a) Uneven illumination and illumination fluctuation of the lamp for illuminating document.
A fluorescent lamp, for example, is used as the lamp for illuminating the document. Partly because the lamp has a definite length and partly because the intensity of light is lower at both ends than at the central portion due to the peculiar light-emitting mechanism. In addition, the fluorescent lamp is gradually darkened at both ends thereof during long use. Furthermore, the illumination distribution is varied depending on the manner of mounting of the lamp.
(b) Loss of light in lenses of optical system
The quantity of light transmitted through a lens is lowered at the peripheral part of the lens in accordance with the cosine biquadratic law. For instance, the quantity of light at the peripheral portion of the lens is as small as 78% when the image half angle is 20.degree..
(c) Lack of uniformity in sensitivity of image sensor
Due to reasons concerning fabrication or production, the image sensor such as the solid state image sensor, e.g. charge coupled device (CCD), and diode array often lack uniformity of sensitivity.
Various countermeasures have been taken hitherto in order to correct the shading. For instance, it has been proposed to use a light disbribution board to reduce the intensity of light at the central portion of the lamp to the same level as that in the end regions thereby to uniformalize the intensity distribution over the entire length of the lamp. This countermeasure, however, is quite invalid against the blackening of the lamp at both end regions, although it is effective in the initial state of use. In order to compensate for the change in the intensity distribution due to blackening, it is necessary to frequently readjust the light distribution board.
It is also proposed, in order to accurately effect the correction, to place an image sensor which outputs the shading waveform in addition to the image sensor for reading the document and to make operation for combining the image signal picked up from the document and the shading waveform. This countermeasure is also unsatisfactory because it cannot make compensation for the fluctuation attributable to the lack of uniformity in sensitivity of the photoelectric element or change in the sensitivity attributable to a change in the ambient temperature, although it can effectively be used for the correction of shading caused by the light source.
Still another correction method employs the steps of illuminating a surface at a uniform illumination, making a photoelectric conversion of the light signal from the surface, conducting an A/D conversion to convert the analog electric signal to digital signal, storing the digital signal in a memory element, and reading the document while correcting the shading by the content of the memory. This method can provide a considerably high accuracy of correction. However, the conversion time of the A/D converter becomes shorter as the driving frequency of the image sensor becomes higher. Thus, ordinary A/D converters cannot satisfactorily cope with the demand for high-speed reading. In addition, it is necessary to employ memory element having larger capacity as the number of picture images is increased.